Digital integrated circuits frequently require voltage comparators as buffers on input lines in order to perform logic-level shifting from one set of logic levels to another, or to accept the differential input signals used in noisy environments. A well-designed voltage comparator will have high gain and low input offset voltage, with the result that sensitive and accurate comparisons can be performed between the input signals. However, it is equally important that the digital output level of the comparator be compatible with the digital logic levels of the IC's internal logic gates.
Ideally the output signal of the voltage comparator will be at a level exactly at the switching point of the internal digital logic when the two inputs to the comparator are at the same voltage, i.e. with no input differential voltage. When one of the differential inputs is raised or lowered, the output of the comparator will respond by switching to a high or low level, typically representative of a digital"1" or "0", respectively. If the level of the output voltage of the comparator is not at the logic switching point when its inputs are at the same voltage, then the output of the comparator will be a skewed version of the differential input. In the presence of small differential input signals, the comparator's output may not make transitions past the logic switching point, and input data will be lost.
Accordingly, for high sensitivity to input signals it is important not only that the input voltage comparator have a low input offset voltage, but also that the output level be centered around the digital logic switching point. This can be difficult to achieve in the presence of supply voltage, temperature, and process variations. All of these variations tend to alter the comparator's output voltage level.
There have been many attempts in the prior art to provide a voltage comparator for responding to input signals for accurately driving integrated circuit logic dates. Fang et al. U.S. Pat. No. 4,371,843 teaches the use of feedback from one side of an input stage of a differential amplifier to drive a single transistor current source.
In a paper authored by L. E. Larson et al., entitled "GaAs DIFFERENTIAL AMPLIFIERS", appearing in the 1985 GaAs IC Symposium Technical Digest, IEEE, pages 19 through 22, the use of level shifting circuits on both sides of a ,differential amplifier is taught. Also, feedback from one side of the input differential stage is used to drive a single transistor current source. Harris U.S. Pat. No. 4,479,094 also teaches a use of a level shifting circuit, but only on one side of a differential amplifier.
In Pengue, U.S. Pat. No. 4,616,189 a differential amplifier is disclosed that includes level shifting circuits for shifting voltages provided at differential outputs of the amplifier down to GaAs compatible output levels. Pengue also teaches the use of a differential current source with common-mode feedback from the input differential pair.